1. Field of the Invention
The present invention relates to the field of electronic radar, and more particularly to apparatus for providing a highly linear ramp modulation for a voltage controlled oscillator (VCO).
2. Description of the Related Art
An frequency modulated continuous wave (FMCW) radar system often uses a linear frequency modulation to provide accurate range information. The resolution of the ranging information is directly dependent on the linearity of the transmit signal. However, the performance output of a voltage controlled oscillator (VCO) used to generate the radar signal is typically non-linear. Therefore, special effort must be taken to insure that the control voltage for the VCO produces the desired linear modulation ramp output slope.
Existing long range millimeter-wave (MMW) FMCW radars utilize surface acoustic wave (SAW) delay line linearizers. SAW delay line systems are not practical for missile and artillery systems due to high-G survivability requirements and size and cost constraints. These devices typically down-converted a MMW VCO to a microwave delay line linearizer, such as a SAW. Other methods down-convert and then divide to a digital or L-C delay line, which exhibits poor discriminator linearity which severely limits the radar range resolution.
U.S. Pat. No. 5,379,001, entitled "Closed Loop Linearizer For Ramp Modulated VCO", issued Jan. 3, 1995 to applicant is directed to a closed loop linearizer which takes a digital count and error correction of time segmented modulation ramp approach. The entire contents of U.S. Pat. No. 5,379,001 are hereby incorporated by reference. While this approach provides improved linearity over open loop techniques (i.e. DDM), there are still several drawbacks to using this counting approach. The digital counting approach offers low cost for high volume applications but does not provide adequate linearity for long range seeker applications.
What is needed is a linearizer which replaces the more expensive higher frequency surface acoustic wave delay system with a lower cost, high-G survivable, small linearizer, while maintaining high linearity.